MLB executive vice president Rob Manfred notified teams of the action Tuesday in a memorandum to club presidents, chief executive officers and general managers.

Nuñez, a consultant for Cabrera's agents at ACES Inc., took responsibility for fabricating a website that the San Francisco Giants All-Star outfielder was relying on as he prepared for a grievance to challenge a drug test that was positive for testosterone.

Cabrera dropped his grievance last week after MLB uncovered the scheme, and he was suspended for 50 games.

"Please be advised that commissioner Selig has directed that all major league clubs are prohibited from granting Juan Carlos Nuñez access to their clubhouses or other nonpublic areas," Manfred wrote in the memo, which was obtained by The Associated Press. "Nuñez is affiliated with ACES Inc. sports agency. Nuñez is currently under investigation for misconduct related to our recent matter under the joint drug program.

"In addition, Nuñez is not certified as a player agent by the Major League Baseball Players Association. Clubs should not conduct contract negotiations with Nuñez or otherwise deal with him regarding players on the 40-man roster."

Seth Levinson, who along with his brother Sam runs ACES, said Nuñez was a paid consultant for the company.

The memo was first reported by ESPN.com. Meanwhile, the New York Daily News reports that investigators from Major League Baseball, traveled to the Dominican Republic, acquired a jar of the alleged substance from the phony website set up by Cabrera's camp, and sent the substance to the World Anti-Doping Agency's testing labs in Utah.

Weeks later it was discovered that the substance was fake.

MLB says lower ratios may trigger more drug tests

Baseball and its players' union say urine samples in drug tests may be subject to additional analysis even if the ratio of testosterone to epitestosterone is under the level that typically is considered a positive.

The joint statement Monday came five days after San Francisco outfielder Melky Cabrera, the All-Star game MVP, was suspended for 50 games for testosterone.

Most drug-testing programs consider a T/E ratio of 4-to-1 or higher to be a positive. But for several years in baseball, the policy has been that samples are potentially subject to further scrutiny if the ratio is below 4-to-1. The usual ratio in adults is 1-to-1.

"The fact that a sample has a T-E ratio below 4:1 does not mean that sample is free from further analysis," the joint statement said. "The characteristics of every sample, including the T/E ratio, are analyzed at the WADA-certified laboratory in Montreal and the laboratory determines which samples are subjected to Carbon Isotope Ratio (CIR) testing. All samples with a T/E ratio above 4:1 are subjected to CIR but so are many samples with a T/E ratio less than 4:1."

The CIR test determines if testosterone came from outside the body.

"The procedures used in the Montreal laboratory make more extensive use of CIR than those used in other labs and there is no doubt that baseball's program detects testosterone positives that other programs would fail to detect," Christiane Ayotte, director of the World Anti-Doping Agency-certified laboratory in Montreal, said in a statement released by MLB and the union.